1. Field of the Invention
The present invention relates to a multi-type air conditioner system with an oil level control for parallel operating compressors and, more particularly, to an air conditioner system having a heat-pump type refrigerating cycle, which includes an outdoor unit having at least two variable-capability compressors and a plurality of indoor units.
2. Description of the Related Art
In general, the above multi-type air conditioner system performs a single-compressor operation involving one of the compressors of the outdoor unit or a parallel operation involving two compressors in accordance with a demand capability of each indoor unit.
The outdoor unit has two variable-capability compressors and two inverter circuits for supplying drive power to these compressors. This outdoor unit is coupled with a branching unit to which a plurality of indoor units are coupled.
With the above design, the individual indoor units send to the branching unit their respective frequency setting signals representing their demand capabilities according to their air conditioning loads. The branching unit acquires the demand capabilities of the individual indoor units from the received frequency setting signals and sends a sum frequency setting signal corresponding to the sum of the demand capabilities, to the outdoor unit. In accordance with the received sum frequency setting signal, the outdoor unit controls the output frequency of each inverter circuit to fulfill the capability requested by each indoor unit.
The present inventors have already disclosed an invention which contributes to an improvement of this type of air conditioner system, in U.S. patent application Ser. No. 225,483, entitled "Multi-Type Air Conditioner System With Starting Control For Parallel Operated Compressor Therein" and filed on July 28, 1988, Great Britain Patent Application No. 8818016.1 and filed on July 25, 1988, Australian Patent Application No. 19792/88. The disclosed invention is concerned with a technique for smooth execution of the above mentioned parallel operation and improvement of oil-balancing effect.
The technique for improving the oil-balancing effect is to provide an oil return passage in the individual compressors having their base sections coupled to each other by an oil-balancing pipe, thereby preventing the undesired locking of the compressors which may be caused by dry-out of the oil.
This technique disclosed in the earlier application is particularly effective in a case where two compressors have the exact (maximum) capability. In practice, however, no two compressors are exactly the same so that their capabilities, though specified to be the same, should differ from each other in strict sense. It is often the case that two compressors having a significant difference in their (maximum) capabilities are combined in the aforementioned air conditioner system, a combination of three-horse power (HP) type and five-horse power (HP) type, for example.
It is therefore necessary to further improve the oil-balancing effect which could be provided by the technique used in the earlier application.
As will be described later, using a large-diameter oil-balancing pipe can provide an oil-balancing effect that copes with the difference in capabilities of two compressors. In this case, however, there would arise a problem of generating undesired vibration or noise that is transmitted to each compressor through the large-diameter oil-balancing pipe. This problem is likely to deteriorate the mechanical strength of this pipe and thus damage or break it at the worst, which is crucial to the air conditioner system in terms of reliability.
In this respect, therefore, it is necessary to consider some means to prevent adverse influence on current release from being caused by the aforementioned improvement of the oil-balancing effect.